1. Field of the Invention
The present invention relates generally to magnetic tape cassettes used for recording digital signals such as PCM (pulse code modulated) signals and the like. More particularly, this invention relates to a novel construction of a magnetic tape cassette in which a spring is positively and easily attached to a front lid mounted pivotally to a cassette housing to close a front opening portion of the cassette housing at which a magnetic tape is exposed.
2. Description of the Prior Art
A PCM recording and/or reproducing apparatus has recently been proposed in which an analog signal, such as an audio signal or the like, is converted to a digital signal, for example a PCM signal, and then recorded on and/or reproduced from a magnetic tape. This PCM recording and/or reproducing apparatus can obtain a relatively high recording density by utilizing a rotary head. When the PCM recording and/or reproducing apparatus is to carry out the recording and/or reproduction by using the rotary head, the magnetic tape is withdrawn from a magnetic tape cassette through a front opening therein, wrapped around a rotary drum on which the rotary head is mounted and then the signal is recorded thereon and/or reproduced therefrom.
However, when oily components such as fingermarks, smudges, dust and so on adhere to the magnetic tape, dropout occurs in the signal reproduced from the magnetic tape. Therefore, it is necessary to enclose the magnetic tape as much as possible so as to protect it from contact with these oily components.
To this end, a type of magnetic tape cassette such as generally shown in FIGS. 1 and 2 has been proposed which has a rotatable front lid to close the front opening of the cassette housing at which the magnetic tape is exposed and through which it is withdrawn. This magnetic tape cassette has a cassette housing 1 which is formed of an upper half 2 and a lower half 3 coupled integrally by screws (not shown). A front lid 4 is pivoted at its side portions 4a to rotary shafts (not shown) on right and left side walls of cassette housing 1 near the front thereof to be freely rotatable. Springs (not shown) attached to the rotary shafts bias front lid 4 in the direction to cover or close the front opening of the cassette housing 1. When this tape cassette is used, front lid 4 is pivoted away from the front of cassette housing 1 to expose a magnetic tape 5 at the front opening.
One example of a prior art magnetic tape cassette of this kind is disclosed in Japanese published utility model application No. 60-17112. In such a magnetic tape cassette, as shown in FIG. 3 of the present application, a coiled portion 7a of a twist coiled spring 7 engages with rotary shaft 6 extending from the inner side surface of side portion 4a of front lid 4, with one end portion 7b of coiled spring 7 engaging front lid 4 and the other end portion 7c of coiled spring 7 engaging with cassette housing 1 to urge front lid 4 to rotate so as to close the front opening of cassette housing 1.
However, in this magnetic tape cassette, coiled spring 7 is engaged only with rotary shaft 6. Accordingly, when in the assembly process of this magnetic tape cassette attaching front lid 4 is attached to cassette housing 1, coiled spring 7 wobbles and rotates around rotary shaft 6 and easily falls off therefrom, making it difficult to engage coiled spring 7 with both front lid 4 and cassette housing 1.
Therefore, in order to remove these defects, another magnetic tape cassette has been proposed and is disclosed in Japanese laid-open utility model application No. 60-166880. FIGS. 4 and 5 are schematic representations illustrating the relevant portion of this previously proposed magnetic tape cassette. As shown therein front lid 4 has a spring engagement portion 8 formed at the projecting end or top of rotary shaft 6. Coiled portion 7a of coiled spring 7 engages rotary shaft 6 between engagement portion 8 and side wall 4a. One end portion 7b of coiled spring 7 is inserted into a concave engagement portion 6a formed on rotary shaft 6 and the other end portion 7c of coiled spring 7 contacts with and engages the upper surface of upper half 2 of cassette housing 1. In addition coiled portion 7a of spring 7 is supported against a spring supporting face 2b formed on a base portion 2a of upper half 2, preventing end portion 7b of spring 7 from coming out of concave portion 6a.
According to the prior art magnetic tape cassette illustrated in FIGS. 4 and 5, during the assembly process when coiled spring 7 is attached to rotary shaft 6 of front lid 4, coiled portion 7a of coiled spring 7 must be expanded along its diameter so as to enable coiled portion 7a to pass beyond spring engagement portion 8 to engage with rotary shaft 6 and end portion 7b of spring 7 must be engaged with concave portion 6a. This requires a significant amount of time and labor in attaching coiled spring 7 to front lid 4. In addition, upper half 2 must be provided with base portion 2a to form spring support face 2b, so that this magnetic tape cassette has a complicated construction and is both heavy and costly.
To avoid the above defects, yet another magnetic tape cassette has been proposed, as shown in FIGS. 6 to 8. FIG. 6 is a side view showing the relevant part of front lid 4 of the magnetic tape cassette, FIG. 7 is a cross-sectional view taken along line A--A' in FIG. 6 and FIG. 8 is a cross-sectional view taken along line B--B' in FIG. 6.
Referring initially to FIG. 6, in this magnetic tape cassette front and rear support members 9a and 9b are formed on the inner surface of side portion 4a of front lid 4 for association with end portion 7b of coiled spring 7. As shown in FIG. 7, front support member 9a projects upwardly in parallel to the inner surface of side portion 4a as an L-shaped member to support end portion 7b of coiled spring 7 at both its lower surface and its side surface. As shown in FIG. 8, rear support member 9b projects perpendicularly to the inner surface of side portion 4a so as to support the upper side surface of end portion 7b of the coiled spring 7, with the upper portion of rear support member 9b being tapering in an inclined plane towards the upper side thereof.
Spring 7 is attached to the front lid 4 as follows. Initially, coiled portion 7a of coiled spring 7 is engaged with rotary shaft 6. At this point, end portion 7b of coiled spring 7 contacts the upper end of front support member 9a, as shown by a broken line in FIG. 6. By pressing at the end of end portion 7b from above, end portion 7b of coiled spring 7 can be slid along the inclined plane of rear support member 9b in the direction shown by arrow D in FIG. 6 to engage with rear support member 9b as shown in FIG. 8.
As a result, both front and rear support members 9a and 9b receive end portion 7b of coiled spring 7 at its lower and upper surfaces respectively to positively attach coiled spring 7 to front lid 4.
However, spring 7 is necessarily very small and short and it is very difficult in practice to attach it to front lid 4 by pressing on the end of end portion 7b from above. In addition, spring 7 must be attached to front lid 4 by two separate movements in different directions. The first is the movement of spring 7 along rotary shaft 6 to attach it thereto and the second is the downward movement for engaging spring 7 with both front and rear support members 9a and 9b as described above. The requirement of two perpendicular movements makes the work of attaching spring 7 very troublesome and in particular creates great difficulties in automating the assembly process of this prior art magnetic tape cassette.